This invention generally relates to materials characterization. More particularly, the invention is directed to methods and devices for measuring the physical properties of articles which may be used under variable conditions, in terms of temperature, stress, and the like.
Many types of materials are used commercially under a variety of conditions. Examples of such materials are polymers (e.g., plastics), ceramics, and metals. As a more specific illustration, many specialized metal components are used in a wide variety of industrial applications, under a diverse set of operating conditions. As an example, the various superalloy components used in turbine engines are exposed to high temperatures, e.g., above about 750° C. Moreover, the alloys may be subjected to repeated temperature cycling, i.e., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating.
Yet another example relates to structural components employed in the nuclear industry, e.g., components formed of stainless steel. These materials are often exposed to very aggressive environments, in terms of heat, corrosion, and the like, which can lead to various levels of cyclic or steady stress. The resulting occurrence is often referred to as “stress corrosion cracking” or “corrosion fatigue”. Damage from stress corrosion cracking is of great concern, since material failure can be very unpredictable.
Clearly, methods for accurately and conveniently measuring the physical properties of these materials are critical for predicting their useful life. Some of the properties and attributes typically measured for material samples are as follows: material temperature, tensile characteristics (e.g., stress, strain), creep characteristics, fatigue, crack initiation and crack growth; and electrical resistivity.
Many different types of equipment and instrumentation can be used to determine these properties. For example, pyrometers, IR cameras, thermistors, and thermocouple thermometers are often used to determine the temperature of metal components like those described above. Extensometers and similar devices are often used to measure the absolute strain or creep in a metal component. Other strain-measuring devices are also used, e.g., bonded metallic strain gauges. Furthermore, various types of mechanical testing machines (e.g., constant displacement-rate types and constant load types) are used to carry out static tension and compression tests on metal components.
The measuring devices mentioned above are often quite suitable for carrying out tests on various test specimens (samples). However, there are some drawbacks associated with their use. For example, many of the devices rely in part on electrical connections to the samples, e.g., various wires which join portions of the samples to different locations on the devices. The electrical connections are often made by welding or brazing techniques. As an example, thermocouples are usually spot-welded to selected sites on a test specimen. These weld and braze connections can sometimes break, e.g., when the test specimen is subjected to high temperatures and/or rough handling.
Braze and weld connections may fail more frequently when the test specimens are subjected to a wide range of temperatures, e.g., about 150° F.-2300° F. (66° C.-1260° C.). Considerable care has to be taken to select the proper braze or weld material for a given temperature environment. Moreover, the connections sometimes have to be carried through fabricated openings in a furnace wall, which can be problematic.
Furthermore, the braze or weld connection may itself cause a disturbance on the surface of a test specimen, which adversely affects the test. For example, sensor wires are sometimes spot-welded on a metallic specimen, to monitor crack initiation and crack growth, e.g., as part of a tensile or fatigue test. The sensor wires are designed and emplaced to detect very minute cracks in selected surface regions (e.g., high stress regions)—usually by detecting changes in voltage or resistance. However, the bonding of the sensor wires to the surface may itself initiate or expand surface cracks, thereby invalidating the test.
Steps can be taken to minimize surface disturbance, e.g., by welding the sensor wires farther away from the region of interest. However, the relocation of the sensor wires may cause other problems. For example, space requirements along the specimen surface may make wire attachment more difficult. Furthermore, complicated adjustments may have to be undertaken to recalculate the stress-electrical equations, compensating for the new wire positions. Moreover, the accuracy of the test may still be compromised somewhat by the re-positioning of the sensor mechanism.
With some of these concerns in mind, new techniques and materials for measuring the properties and attributes of various samples and test specimens would be welcome in the art. New sensor materials—e.g., those which connect a test specimen to a particular type of measuring device—would be especially useful in situations where conventional sensor connections may cause the problems described above. Moreover, testing devices which include sensor materials whose size, shape, and composition can be readily adjusted would be of great interest in the art.